Two volume pump



Oct. 13, 1964 B. H. PAULY TWO VOLUME PUMP Filed Dec. 29, 1961 INVENTOR. 5120c: H-PAUL Y iE/CHEK 5 N w. w M W MW 7 A A my N1 u m MW United States Patent 3,152,555 TWO VOLUME PUMT Bruce H. Pauly, Chagrin Falls, Ohio, assignor to The Weatherhead Company, Cleveland, Ohio, a corporation of Ohio Filed Dec. 29, 1961, Ser. No. 163,222 3 Claims. (Cl. 103173) This invention relates generally to variable positive displacement pumps of the reciprocating piston type and more particularly to variable displacement controls for pumps of this type.

In co-pending applications of Tadeusz Budzich which are owned by the assignee of the instant application, namely, in applications Serial No. 825,005, filed July 6, 1959, now Reissue Patent No. 25,553, and Serial No. 847,512, filed October 20, 1959, now Patent No. 3,117,- 524, there is disclosed a pump having a housing in which a cylinder block is mounted for axial reciprocating movement. The cylinder block has a plurality of pumping cylinders extending therethrough parallel to the axis of reciprocating and pistons are mounted within each of the cylinder bores and reciprocated by a wobble plate type drive mechanism. Fluid in the interior of the pump housing flows through a filling slot or port in the cylinder block which opens into the cylinder bore. The port is uncovered by the piston during the rearward portion of the stroke to allow fluid to enter and fill the cylinder bore. As the piston moves forward, the fluid within the cylinder bore is forced back outward through the filling slot until the piston covers and seals off this port, after which the remainder of the fluid within the cylinder bore is forced outward past a check value into an outlet or discharge port. The effective displacement of this pump is controlled by axial movement of the cylinder block to vary the point during the piston stroke at which the port is sealed off. Thus, the effective displacement of the pump can be varied from a maximum, substantially equal to the displacement of all of the piston strokes, down to zero.

These said co-pending applications of Tadeusz Budzich also disclose a control mechansim adapted to shift the cylinder block responsive to the pressure in the outlet port so as to vary the eifective displacement of the pump in order to maintain a substantially constant outlet pressure. Thus while the effective output displacement or volume of fluid is varied depending upon the load on the hydraulic system, the outlet pressure remains constant at the regulated pressure level unless the demand should exceed the maximum capacity of the pump.

It is a primary object of the present invention to provide a variable displacement pump having a pressure responsive control system operable to reduce the effective displacement of the pump to a predetermined level less than maximum in response to a pressure less than the maximum regulated output pressure of the pump and maintain this reduced displacement independently of further increase in outlet pressure.

It is another object of the present invention to provide a two volume positive displacement pump operable to produce a high volume at low pressure and a relatively low volume at high pressure in which the changes of the pump characteristics are dependent upon the demand placed upon the system.

It is another object of the present invention to provide a pump as set forth in the preceding object having a first pressure responsive control adapted to reduce the effec tive output volume from the high volume level to a low volume level responsive to an increase in outlet pressure beyond a relatively low pressure level, and having a second pressure responsive control operable to limit the maximum output pressure of the pump by reducing the vide a pump as set forth in the preceding object in which the second control is operable independently of the first control to provide a positive maximum pressure limiting control independent of the output volume of the pump and independent of the operation of the first control.

Further objects and advantages of the present invention will readily become apparent to those skilled in the art upon a more complete understanding of the preferred embodiment of the invention as shown in the drawing which is a longitudinal cross-sectional view of a pump incorporating the present invention.

The pump includes a pump housing 10 which is generally cylindrical and encloses a fluid chamber 11 Within which the pump mechanism is located. An inlet 12 is formed on the wall of the pump housing to permit fluid to enter and fill the chamber 11 from a suitable reservoir. At one end, the pump housing 10 is closed ofi by an end plate 14 over which is fitted an end cap 15. The end cap 15 and the end plate 14 are secured to the pump housing 10 by means of suitable cap screws 16. End cap 15 is provided with an outlet chamber 18 for connection with the remainder of the hydraulic system.

At the opposite end, the pump housing 10 has an end wall 21 having a centrally located opening 22 therein through which projects a drive shaft 24. Drive shaft 24 extends through a drive member 26 to which it is non rotatably secured by means of the spline connection 25. The drive member 26 is rotatably journaled in a bearing assembly 27 mounted on the end wall 21 and has an oblique face 29 from which projects a hub 30. A wobble plate 32 is mounted on oblique face 29 and hub 30 by means of bearings 31. Thus as the drive member 26 is rotated by the drive shaft 24, the movement of the oblique face 29 and hub 30 causes wobble plate 32 to oscillate about the axis of the drive shaft. To prevent the wobble plate 32 from rotating with respect to the pump housing 1% it is provided with a projecting stud 34 on which is rotatably journaled a guide block 35. Guide block 35 engages the sides of a channel member 36 mounted within the pump housing. The guide block 35 is thus restrained to reciprocating sliding movement along the axis of the channel member 36 and positively restrains any rotation of the wobble plate 32.

A tubular guide 40 having a hollow bore 41 concentrically disposed therein projects from the end plate 14 coaxial with the drive shaft 24. At the outer end of the tubular guide 40 there is mounted a stop plate 43 having a hollow shank 44 fitted within the end of bore 41. A bearing bushing 46 is fitted within the hollow shank 44 to journal the pilot end portion 47 of the drive shaft 24. A cylinder block 50 is slidably journaled on the tubular guide 40 for axial reciprocating movement between the stop plate 43 and the end plate 14. To prevent rotation of the cylinder block 50 on the tubular guide 40, the cylinder block is provided with a pin 52 which engages a longitudinal slot 53 formed on the outer surface of the tubular guide 4!).

The cylinder block 50 has a plurality of cylinder bores 55 disposed equidistantly around the cylinder block. These cylinder bores 55 are of uniform diameter and extend from end to end through the cylinder block parallel to the axis. A filling slot 56 is formed on the outer periphcry of cylinder block 50 intermediate the ends and opens into each of the cylinder bores 55 to serve as an inlet port for admitting fluid from the pump chamber 11. Since all of the cylinder bores and their associated parts are identical in structure and operation, only one cylinder bore is shown in the drawing and described in detail hereinafter.

A piston 58 is fitted within the cylinder bore 55 at the end adjacent the stop plate 43. Piston 58 has a tubular skirt portion 59 which extends outward through a suitable opening in the stop plate 43 toward the wobble plate 32. The piston has a head portion 60 which extends to a position adjacent the filling slot or port 56 when the piston is in the retracted position and the cylinder block is in the maximum output volume position adjacent stop plate 43 as shown in the drawing. A helical compression spring 62 surrounds the piston skirt 59 and abuts at one end against the stop plate 43 and at the other end against a retainer ring 63 secured on the end of the piston skirt. A push rod 65 is positioned within the tubular piston skirt 59 and has a ball end portion 66 bearing on the underside of the piston head 60. The other end of push rod 65 is formed into another ball end portion 67 which fits within a cup-like recess 68 on the wobble plate 32. Thus it will be seen that through rotation of the drive shaft 24 and drive member 26, the wobble 32 is oscillated to reciprocate the piston 58 within the cylinder bore 55 with a sinusoidal motion.

A reaction piston 70 is fitted within the end of cylinder bore 55 opposite that containing the piston 58. The reaction piston 70 has a hollow bore 71 extending therethrough to conduct fluid from the interior of cylinder bore 55. The reaction piston 70 extends into a counterbore 72 within the end plate 14 where it makes surface abutting contact with a port member 75 secured within a chamber 74 formed in the end cap 15. A helical compression spring 77 surrounds the reaction piston 70 and abuts at one end against the cylinder block 55 and at the other end against a spring retainer 78 secured on the end of reaction piston 70 adjacent the port member 75. Thus the compression spring 77 not only serves to bias the reaction piston into sealing contact with the port member 75, but also by its reaction force biases the cylinder block 50 toward the stop plate 43. It will therefore be observed that the cylinder block 50 is biased in this direction by a force equal to the sum of the forces of all of the compression springs surrounding the reaction pistons associated with the other cylinder bores in the cylinder block.

The port member 75 has a bore 80 which is axially aligned with the bore 71 in the reaction piston 70. The end of bore 80 away from reaction piston 70 is closed off by a check valve plate 81 biased against the port member by a compression spring 82 supported within a cage 83 in the chamber 74. A passage 84 leads from the chamber 74 to the outlet chamber 18. From the above it will be seen that as the piston 58 moves toward the reaction piston 70 on its forward or pumping stroke, the fluid within the cylinder bore is forced through the bore 71 in the reaction piston, past the check valve plate 81, into chamber 74 and thence through passageway 84 to the outlet chamber 18.

The effective output volume of the pump is controlled by shifting the axial position of the cylinder block 50 and hence the filling slot or inlet port 56 with respect to the pump housing. As the piston 58 moves forward, so long as the port 56 remains uncovered by the piston head 60, the fluid within the chamber will be forced out through the port 56 into pump chamber 11 since the pressure within the cylinder is insufficient to shift the check valve plate 81 away from the port member 75 under the biasing force of spring 82. After the piston head 60 passes the port 56, the port is sealed ofl? and the remaining portion of the piston stroke forces the fluid within the cylinder bore past the check valve into the outlet chamber. Since no fluid is pumped into the outlet chamber 18 until the port 56 is covered, movement of cylinder block 50 to shift the position of the port 56 relative to the piston stroke will vary the effective output volume of the pump.

As shown in the drawing, with the cylinder block in the position adjacent stop plate 43, the port 56 will be closed during the initial portion of the stroke and substantially the entire length of the piston stroke is employed for pumping fluid into outlet chamber 18. However, when the cylinder block 50 is shifted toward the end plate 14 so that the port 56 is closed by the piston head only at the very end of the piston stroke, or is not closed oif at all, substantially all of the fluid pumped by movement of the piston 58 passes back into the pump chamber 11 and no fluid is pumped into the outlet port 18. Thus the axial position of cylinder block 50 determines the effective output volume of the pump and this output volume may be varied from a maximum down to zero by means of suitable controls for positioning the cylinder block.

One of the controls for shifting the position of the cylinder block 50 is mounted within the end of plate 14 and the tubular guide 40 and its method of operation is described in greater detail in said co-pending applications referred to hereinabove. The end plate 14 has a neck portion 87 which extends into the outlet chamber 18, from which it is sealed around its outer periphery by a suitable O-ring seal 88. An axial bore 90 extends through the neck portion 87 and end plate 14 between the outlet chamber 18 and the bore 41 within tubular guide 40. A valve spool 92 is slidably journaled within axial bore 90 and is provided with an end portion 93 exposed to the pressure within outlet chamber 18. Valve spool 92 also has outer and inner annular grooves 94 and 95, respectively, which define a land portion 96 therebetween. A passage 98 extends obliquely through the neck portion 87 to connect the outlet chamber 18 with the outer annular groove 94. An annular port 100 is formed in the neck portion 87 around the land 96 and is connected by a passage 101 to the bore 41 in tubular guide 40. A drain passage 102 extends through the end plate 14 between the inner annular groove and the pump housing chamber 11.

The valve spool 92 extends into a chamber portion 103 of bore 41 within tubular guide 40 where it is secured to a spring abutment 104. A helical compression control spring 105 is positioned within chamber 103 to abut at one end against the spring abutment 104 and at the other end against a plug 106 which seals off the other end of chamber 103 and is held in place in bore 41 by a suitable snap ring 107. A radial bore 108 extends through the wall of tubular guide 40 to connect the chamber 103 with a reduced passage portion 109 formed on the outer periphery of tubular guide 40. Reduced passage portion 109 communicates with a counterbore 111 formed on the cylinder block 50 about the tubular guide 40. A chamber 112 is formed within the counterbore 111 and closed off by means of an annular piston 114 fixedly secured on the tubular guide 40 by a snap ring 115.

The operation of this control is as follows. The valve spool 92 is normally biased toward the outlet chamber 18 by the force of compression spring 105 and the force of fluid pressure within chamber 103 acting on the effective cross-sectional area of the valve spool. Chamber 103 is connected through passage 101 to the annular port 100, and since the valve spool 92 is in a leftward position, the land 96 has shifted so that the annular port is connected to the inner groove 95, which in turn is connected to the pump housing chamber 11 by drain passage 102. The pressure within the chamber 103 is determined by the force applied to the cylinder block 50 by the compression springs 77 tending to reduce the volume of chamber 112 and force the fluid into chamber 103 and hence back into the pump housing chamber 11. This insures that when there is substantially no pressure in the outlet chamber 18, the cylinder block 50 will be in the right-hand or maximum displacement position indicated in the drawmg.

When the pump is in operation and fluid pressure builds up within the outlet chamber 18, this pressure acts upon the exposed valve spool end 93 and tends to force the valve spool 92 toward the right against the force of compression or control spring 105 and the fluid pressure within chamber 103. When the outlet chamber pressure exceeds a predetermined level set by the force of the control spring 105, the valve spool 92 will be shifted into the right-hand position in which the outer groove 94 is in communication with the annular port 100. In this position, the relatively high pressure in outlet chamber 18 will flow through passage 98 to the outer annular groove 94 and from there into annular port 100 and through passage 101 into chamber 103. The fluid will then pass from passage 103 through the radial bore or port 108 past the reduced passage portion 109 into the chamber 112. The fluid pressure within chamber 112 then acts upon the effective area of counterbore 111 to shift the cylinder block 50 toward the end plate 14 against the bias of the compression springs 77. Since the fluid pressure within the outlet chamber 18 is quite high as compared to the force being exerted by the compression springs 77, the cylinder block 50 will move in that direction to reduce the effective output volume of the pump in the aforedescribed manner. When the effective output volume is reduced so that the outlet chamber pressure returns to the predetermined level, the force of the compression spring 105 and the fluid pressure within chambers 103 and 112 will shift the valve spool 92 back to the neutral position shown in the drawing in which the annular port 100 is blocked off from the land 96 and fluid neither enters nor leaves the chambers 103 and 112.

The pump is provided with a second control mecha nism for shifting the position of the cylinder block 50 for varying the effective output volume of the pump. A rod 120 extends parallel to the pump axis in the lower por: tion of the pump housing and is slidably journaled in a bearing 121 at the end adjacent the wobble plate 32. Rod 120 extends through an opening in end plate 14 into a bore 123 in end cap 15 Where it is slidably journaled in a second bearing 122. A yoke 125 is fixedly secured to the sliding rod 120 adjacent the bearing 121 and extends around a reduced portion 127 of the cylinder block 50. An abutment shoulder 128 is formed on the cylinder block 50 adjacent the reduced portion 127 to bear against the yoke 125.

A plug 130 is threadedly secured in the outer end of bore 123 in end cap 15. A connecting rod 132 is secured to sliding rod 120 by a suitable pin 131 and extends into the hollow bore 135 in plug 130 where it is journaled in a bearing bushing 133. The connecting rod 132 has a piston portion 136 slidably journaled in bore 135 and making sealing contact therewith by an O-ring seal 137. A port 139 is provided in plug 130 to communicate with bore 135 on the inner side of piston 136. Suitable piping 140 connects the port 139 to the outlet chamber 18 so that the pressure within outlet chamber 18 is at all times communicated directly to port 139 to exert a force on the underside of piston 136. W

It will be seen that the piston 136 presents an effective area equal to the differential area between the bore 135 and the connecting rod 132. The pressure acting upon this area tends to force the connecting rod 132 and hence sliding rod 120 toward the left as shown in the drawing. Movement of the rod 120 in this direction causes the yoke 125 to bear against the shoulder 12S and force the cylinder block 50 toward the end plate 14 against the bias of the compression springs 77. It will thus be seen that the movement of sliding rod 120 and hence cylinder block 50 is dependent solely upon the balance of the fluid pressure force within bore 135 and the biasing force of the springs 77. Since the yoke 125 bears only against the abutment 128, movement of rod 120 is operable only to shift the cylinder block 50 toward the end cap 6 14 and is inoperable to shift the cylinder block 50 in the opposite direction toward the stop plate 43.

When the pump is initially started, there will be no fluid pressure within the outlet chamber 18. The cylinder block 50 will then be in the maximum output volume position shown in the drawing. The pump will continue to deliver maximum output volume as the pressure builds up, and when the outlet pressure reaches a first predetermined value as determined by the effective area of piston 136 and the biasing force of the compression spring 77, the piston 136 will be forced toward the left pulling the sliding rod in that direction unitl it abuts against the plug 130. This movement causes the yoke to shift the cylinder block 50 is a leftward direction to reduce the effective output volume of the pump. Since the compression springs 77 are under a relatively high preload while having a relatively low spring rate, the piston 136 will not move until the pump outlet pressure reaches the predetermined level of, for example, 450 p.s.i. When the pressure exceeds this level, the cylinder block 50 will be moved to the left until the rod 120 abuts against the plug when the outlet pressure reaches the level of, for example, 500 p.s.i. With the cylinder block 50 in this position, further increases in the outlet pressure will not cause any further movement of the sliding rod 120 and yoke 125 since movement in that direction is limited by the abutting engagement between the rod 120 and plug 130. So long as the pressure level in the output 18 remains above the level of 500 p.s.i., the yoke will maintain this position and prevent movement of the cylinder block toward the right in the direction of increase of pump displacement. Thus, this second control operates to reduce the effective output volume to a level substantially less than maximum output volume whenever the output pressure exceeds a predetermined pressure level. The cylinder block 50 will then remain in this position and deliver the intermediate volume until the pressure builds up to a relatively high level where the first control becomes operative. Because the movement of the valve spool 92 is controlled by the pressure of the control spring 105, this control may be set to operate at a high pressure level of, for example, 3000 p.s.i. If the outlet pressure exceeds this level, the valve spool will be shifted in the aforedescribed manner to cause fluid pressure to shift the cylinder block 50 toward the end plate 14 to reduce the effective output volume of the pump and hence reduce the pressure in outlet chamber 18 to the high pressure level. Thus the control employing the valve spool 92 is operable to vary the output volume at a pressure level of 3000 p.s.i. between the intermediate output volume level determined by the position of yoke 125 and zero output volume.

A pump incorporating these controls will provide a high output volume at relatively low pressure and a low output volume at relatively high pressure. A pump with these characteristics is particularly useful for applications such as a swaging press where a rapid approach of the ram at high speed is desirable and requires little force. After the ram reaches a position close to or engages the work, the movement should be relatively slow and exert a high force. With the present pump, a level of 500 p.s.i. may be selected to provide a maximum volume of, for example, 40 gallons per minute for the approach. When the ram contacts the workpiece, the pressure will increase thereby bringing the second described control into effect to shift the yoke 125 and reduce the output volume to a maximum of, for example, 10 gallons per minute. The output volume of the pump will then remain constant at 10 gallons per minute until the pressure increases to the level of 3000 p.s.i., after which the first control will operate to shift the cylinder block to vary the output volume between zero and 10 gallons per minute and prevent the outlet pressure from exceeding the 3000 p.s.i. level. Of course it is understood that the values given above may be varied as required for any particular application of the pump. Thus the operating pressure level of the second pump may be varied by changing the preload on helical compression spring 77 and varying the efiective area of the piston 136. The operating level of the first control may also be varied by changing the compression on the control spring 105 and with relatively narrow limits by changing the eflective area of counterbore 111 and the preload of compression spring 77. The values given herein and the application to a hydraulic press are merely by way of example and are not to be construed in any manner as limiting the scope of applicants invention. It will be appreciated that various modifications and rearrangements can be made by those skilled in the art without departing from the scope of the invention as defined in the appended claims.

What is claimed is:

1. A pump comprising a pump housing providing a fluid chamber therein, an inlet port to said fluid chamber, an outlet port on said pump housing, a cylinder block mounted for slidable movement within said fluid chamber along a longitudinal axis, a plurality of cylinder bores in said cylinder block, a piston within each of said cylinder bores, drive means in said pump housing to reciprocate said pistons between forward and retracted positions, means connecting each of said cylinder bores to said outlet port, an inlet port for each of said cylinder bores in said cylinder block, means biasing said cylinder block to the maximum output volume position, first control means including a first fluid motor connected directly to said outlet port to exert a biasing force proportional to the pressure at said outlet port on said cylinder block in opposition to said biasing means, said first control means being operable responsive to fluid pressure in said outlet port above a first pressure level to shift said cylinder block to reduce the output volume, means stopping said first fluid motor to prevent said first fluid motor from moving said cylinder block beyond an intermediate output volume position, and second control means independent of said first control means including a second fluid motor operable responsive to pressure in said outlet port substantially exceeding said first pressure level to vary the effective output volume of the pump to maintain a constant outlet pressure at a second pressure level greater than said first pressure level by shifting said cylinder block between said intermediate output volume position determined by said first control means and the zero output volume position.

2. A pump comprising a pump housing providing a fluid chamber therein, an inlet port to said fluid chamber, an outlet port on said pump housing, a cylinder block mounted for slidable movement within said fluid chamber along a longitudinal axis, a plurality of cylinder bores extending from end to end through said cylinder block parallel to said longitudinal axis, a piston within one end of each of said cylinder bores, drive means in said pump housing to reciprocate said pistons between forward and retracted positions, means within the other ends of said cylinder bores connecting each of said cylinder bores to said outlet port, an inlet port for each of said cylinder bores in said cyilnder block intermediate the ends, means biasing said cylinder block to the maximum output volume position, first control means including piston and cylinder means in said pump housing connected directly to said outlet port to exert a biasing force proportional to the pressure at said outlet port on said cylinder block in opposition to said biasing means, said first control means being adapted to shift said cylinder block toward a reduced output volume position in response to fluid pressure in said outlet port exceeding a first pressure level, stop means for said piston and cylinder means to prevent movement of said piston and cylinder means and said cylinder block beyond an intermediate output volume position less than maximum output volume, and second control means independent of said first control means responsive to fluid pressure in said outlet port substantially exceeding said first pressure level to shift said cylinder block to vary the effective output volume of the pump to maintain the pressure in said outlet port at a second pressure level greater than said first pressure level by shifting said cylinder block between said intermediate output volume position and the zero output volume position.

3. A pump comprising a pump housing providing a fluid chamber therein, an inlet port to said fluid chamber, an outlet port on said pump housing, a guide member in said pump housing extending along a longitudinal axis, a cylinder block mounted for slidable movement within said fluid chamber along said guide member, a plurality of cylinder bores extending from end to end through said cylinder block parallel to said longitudinal axis, a piston within one end of each of said cylinder bores, drive means in said pump housing to reciprocate said pistons between forward and retracted positions, means within the other ends of said cylinder bores connecting each of said cylinder bores to said outlet port, and an inlet port for each of said cylinder bores in said cylinder block intermediate the ends, means biasing said cylinder block to the maximum output volume position, first control means including first piston and cylinder means in said pump housing parallel to said longitudinal axis, a yoke shiftable by said first piston and cylinder means and engageable with said cylinder block to shift said cylinder block toward a reduced output volume position in response to fluid pressure in said outlet port exceeding a first pressure level, stop means for said first piston and cylinder means to limit movement of said piston and cylinder means and said cylinder block to position said cylinder block at an intermediate output volume position less than maximum output volume, and second control means including a second piston and cylinder means coaxial with said guide members independent of said first control means responsive to fluid pressure in said outlet port to shift said cylinder block to reduce the effective output volume of the pump to maintain the pressure in said outlet port at a second pressure level greater than said first pressure level by shifting said cylinder block between said intermediate output volume position and the zero output volume position.

References Cited in the file of this patent UNITED STATES PATENTS 2,344,517 Schnell Mar. 21, 1944 2,669,935 Tucker Feb. 23, 1954 2,882,863 Newton Apr. 21, 1959 2,990,781 Tuck et al. a- July 4, 1961 3,051,092 Lambeck Aug. 28, 1962 

1. A PUMP COMPRISING A PUMP HOUSING PROVIDING A FLUID CHAMBER THEREIN, AN INLET PORT TO SAID FLUID CHAMBER, AN OUTLET PORT ON SAID PUMP HOUSING, A CYLINDER BLOCK MOUNTED FOR SLIDABLE MOVEMENT WITHIN SAID FLUID CHAMBER ALONG A LONGITUDINAL AXIS, A PLURALITY OF CYLINDER BORES IN SAID CYLINDER BLOCK, A PISTON WITHIN EACH OF SAID CYLINDER BORES, DRIVE MEANS IN SAID PUMP HOUSING TO RECIPROCATE SAID PISTONS BETWEEN FORWARD AND RETRACTED POSITIONS, MEANS CONNECTING EACH OF SAID CYLINDER BORES TO SAID OUTLET PORT, AN INLET PORT FOR EACH OF SAID CYLINDER BORES IN SAID CYLINDER BLOCK, MEANS BIASING SAID CYLINDER BLOCK TO THE MAXIMUM OUTPUT VOLUME POSITION, FIRST CONTROL MEANS INCLUDING A FIRST FLUID MOTOR CONNECTED DIRECTLY TO SAID OUTLET PORT TO EXERT A BIASING FORCE PROPORTIONAL TO THE PRESSURE AT SAID OUTLET PORT ON SAID CYLINDER BLOCK IN OPPOSITION TO SAID BIASING MEANS, SAID FIRST CONTROL MEANS BEING OPERABLE RESPONSIVE TO FLUID PRESSURE IN SAID OUTLET PORT ABOVE A FIRST PRESSURE LEVEL TO SHIFT SAID CYLINDER BLOCK TO REDUCE THE OUTPUT VOLUME, MEANS STOPPING SAID FIRST FLUID MOTOR TO PREVENT SAID FIRST FLUID MOTOR FROM MOVING SAID CYLINDER BLOCK BEYOND AN INTERMEDIATE OUTPUT VOLUME POSITION, AND SECOND CONTROL MEANS INDEPENDENT OF SAID FIRST CONTROL MEANS INCLUDING A SECOND FLUID MOTOR OPERABLE RESPONSIVE TO PRESSURE IN SAID OUTLET PORT SUBSTANTIALLY EXCEEDING SAID FIRST PRESSURE LEVEL TO VARY THE EFFECTIVE OUTPUT VOLUME OF THE PUMP TO MAINTAIN A CONSTANT OUTLET PRESSURE AT A SECOND PRESSURE LEVEL GREATER THAN SAID FIRST PRESSURE LEVEL BY SHIFTING SAID CYLINDER BLOCK BETWEEN SAID INTERMEDIATE OUTPUT VOLUME POSITION DETERMINED BY SAID FIRST CONTROL MEANS AND THE ZERO OUTPUT VOLUME POSITION. 